Method of making electrically insulated heating units



Aug. 28,. 1962 R. 1.. SCHWING 3,050,833

METHOD OF MAKING ELECTRICALLY INSULATED HEATING UNITS Filed May 19, 1958 a! INVENTOR.

United States Patent Filed May 19, 1958, Scr. No. 736,393 9 Claims. (Cl. 29-15564) My invention relates to the manufacture of electric heating elements and insulated resistors. More particularly, this invention relates to methods for the construction of resistor type heating elements for various heating applications including use in electrical apparatus.

In electrical heating apparatus, it has often been found advisable to encase or embed conductive resistance elements within an electrical insulating, heat conducting material .to prevent contact with the heating element by electrically conducting materials, to prevent any movement of the resistance elements and/ or for other reasons and thereby to avoid short circuits and shocks, increase the life of the resistance element, and make the resistance element easier to handle.

The insulating or embedding material should have a high resistance to electrical conductivity and good conductivity as to heat. It is also important that its conductivity to heat be uniform so as to maintain a uniform temperature throughout the heating element and thereby prevent overheating of the resistance element locally.

Heretofore, some embedding materials have been found objectionable in that being of a hygroscopic nature their dielectric strength, even if originally good, rapidly decreased with increased moisture content as picked up in use and objectionable current leakage occurred.

It has been known to those experienced in the art that when such materials as magnesium oxide, aluminum oxide, and the like are introduced around a resistor element within a suitable sheath of metal and the resulting assembly subjected to very high pressure by swaging, pressing or rolling, a compact stone-like material with excellent insulating properties will result. The major objection to this method of fabrication lies in the damage to the resistor material resulting from impressing of the small hard sand-like grains into the resistor material surface with resultant change in resistance and a shorter material life. Such materials found satisfactory for coil embedment require compacting under high pressure about the resistance element and heavy, expensive presses or swaging machines must be used for such action. Under such extreme pressure, sharp particles of the embedding material are sometimes forced into the resistor element resulting in pitting its surface with consequent reduction of its cross section to provide possibility of early burn-outs.

It is also known that certain types of heating elements, such as used in electric coffee makers, can not be adequately pressed, rolled or swaged without serious damage to the resistance element and the porcelain core on which the resistance element normally is wound. In most cases where such types of heating elements were desired, resort has been made to the use of porcelain-like cements containing Various amounts of highly hygroscopic sodium silicate, or similar materials as a binder. These cements, under high humidity conditions and high heater operating temperatures, have increasingly poorer insulation values resulting in excessively high current leakage and danger from electric shock.

The principal object of my invention is to provide a novel method, by use of chemical means, for the production of a solid embedment for electrical heater elements, which embedment is inexpensive, of high electrical insulating and extremely low hygroscopic properties and which is produced without in any way damaging the resistance wire of similar heating member.

Another object of my invention is to provide an embedment material which is highly heat conductive, highly resistive to electricity and one which will maintain high dielectric strength over a long service life.

A further object is to provide a thin wall of solid embed-ment material for an electric heater, which material has a low thermal inertia and very rapid heat transfer.

Another object of my invention is to provide a method by which an inexpensive, efiicient heater of any molded shape can be made and which has high watts density without any encasement, sheath or enclosure being required; the heater being used for certain applications such as for radiant heaters, and the like.

Another important object of the present invention is to encompass a resistor unit positioned in a metal receptacle in a powdered sand-like material, such as magnesium oxide or the like, and to treat it with phosphoric acid whereby magnesium phosphate is formed in situ around the resistor unit and a metal phosphate is formed on the inner surface of the receptacle whereby an intimate bond is formed between the inner surface of such receptacle and the magnesium phosphate in the receptacle, and a very efiiective, intimate contact is formed between the particles of the magnesium phosphate for effective heat transfer from particle to particle thereof.

Still another important feature of the present invention is that by practice of the present embedment forming method, an effective control can be provided for the hardness of the embedment material, which hardness is varied by the strength and amount of acid used in practice of the new process of the invention.

Another object of the invention is to provide a method of embedding a resistance element in a stone-like insulating jacket wherein water initially formed in such jacket is removed by heating the resistance element and heater for a relatively long time at a temperature below the boiling point of water to drive oft Water from the hard embedment without any violent evaporation action being produced.

The present invention, as a further object, aims to overcome the foregoing and other difiiculties by eliminating the use of presses and swaging apparatus in electrical heater encasement action by forming a stone-like embedment in situ by chemical action.

In the drawings accompanying my application, I have shown, for purpose of illustration, several well known types of heating elements. Numerous other applications and uses will become evident without departing from the spirit and scope of this invention.

FIGURE 1 shows a conventional tubular type heater element.

FIGURE 2 shows a cross section of the tubular element as taken on line 2-2 of 'FIGURE 1.

FIGURE 3 shows a cross section of a molded embedded element ready for insertion into a metal sheath.

FIGURE 4 shows a cross section of another molded embedded element for use as radiant or infra-red type heaters.

FIGURE 5 shows a partial cross section of a refractory brick type heater element in which the resistor is embedded.

FIGURE 6 is a longitudinal cross section of a typical low inertia type of heating element such as may be used in a coffee making appliance or hot water maker.

FIGURE 7 is a longitudinal cross section of an immersion type cylindrical heater of low thermal inertia.

FIGURE 8 is an enlarged vertical cross section of yet a further type of a heating element of the invention.

With reference to the drawings, and first to FIGURES 1 and 2, thereof, these show a tubular type heating element comprising a helically coiled resistance element 2 extending axially through a tubular metal sheath 3, about the resistance element. The sheath is filled with an electrical insulating material 1. The material 1 has, heretofore, been introduced within the sheath in powdered or granular form and the sheath has been compressed or swaged thereabout to compact the insulating material.

In making such a heating element according to the present invention, preferably magnesium oxide is introduced within the sheath in powdered or granular form, as in the prior methods, and which includes compacting the material by vibration.

In making the heating element proper, I have found that after filling the tube, mold, case or shell with magnesium oxide 1 to completely envelope the resistor element 2 within the mold, casing or shell 3 in the standard methods employed in the art, the introduction of a water solution of phosphoric acid will result in a chemical reaction which changes the magnesium oxide to a solid stone-like mass of magnesium phosphate without any further compaction by swaging, rolling or pressing under high pressures. During the reaction considerable heat is developed tending to drive off some of the water of reaction and water of solution, the balance of which is removed by subsequent application of suctional forces to the heater and/ or by low temperature baking, or heat- The magnesium oxide used in practice of the invention must be electrical grade fused and ground magnesium oxide in the form of small particles called, for the purpose of description, sand.

In practice, it has been found that several of the metallic oxides singly and/or in combination will react with phosphoric acid to form a solid stone-like phosphate mass of high insulating quality and with very low water absorption. Thus, for example, aluminum oxide, cuprous oxide, or Zinc oxide may be used in place of the magnesium oxide, or in combination therewith for practice of the present invention.

In addition, the introduction of phosphoric acid Within the metallic cases or sheath materials, such as 3 in FIG- URE 1, and 17 in FIGURE 6, produces a reaction to form a phosphate film 4 on the inner surface of the sheath or case. This film is of high electrical insulating and heat conducting nature and tends to bond with the phosphate embedment 1 to form .a homogeneous, compact mass of slightly porous character.

When adding the phosphoric acid to the magnesium oxide, ordinary commercial 85% phosphoric acid, commonly known as orthophosphoric acid, is usually diluted. The commercial acid, in one example of the invention, is diluted with about 2 /2 times the volume of acid used of relatively pure water. In practice of the invention, the use of distilled water is preferable, but any relatively pure water can be used insolong as the water does not have an objectionable percentage of dissolved minerals therein, such as would effect the electrical conductivity or resulting electrical insulating properties of the end structure produced. This dilute acid is used, in the example of the invention, in the percentage of approximately cc. of the dilute acid solution to a heating element having 10 cc. of oxide sand therein. The acid solution relatively readily flows through the magnesium oxide by gravity and by capillary action. In all instances, excess acid over the stoichiometric amount is provided. Light or small amounts of air pressure can be applied to the acid solution to aid in forcing it into and through the magnesium oxide grains. Inasmuch as the chemical reaction between the acid and magnesium oxide, or other material used is not complete for some time, the acid solution readily will saturate any given compactment around a heater unit so that complete chemical action can be secured between all of the oxide present and the acid used. After the magnesium oxide is completely saturated and the reaction well advanced, in about six to ten minutes the material has started to set up and the excess acid can be removed, along with some water of reaction and water of solution, by use, for example, of a suitable suction device, such as an aspirator.

The chemical reactions occurring are believed to be as follows:

It is believed that the resistance wire used is actually protected by the embedment material formed therearound, as resistance wire, which usually is made from approximately percent nickel and 20 percent chrome does not bond with and is not externally effected by the phosphoric acid process used. This embedment produced protects the resistance wire from chromium loss due to oxidation, which loss occurs when magnesium oxide particles alone are used for the embedment materials. By forming the subtantially stone-like embedment, the usable life of the resistance wire is thus prolonged. Naturally the hard embedment produced holds the convolutions of the resistance wire separate from each other and prevents any possible creeping of these wires into short circuiting engagement with each other.

It should be understood that the speed of action of the acid in reacting with the oxide used can be varied by controlling the concentration of acid used and that the strengths referred to hereinabove are used for example only but that such strength acid is very effective commercially in that it can give a controllable reaction with the oxide.

In FIGURE 3, 5 is a molded phosphate mass around one or more heating resistors 6. I have found that any adequate type of mold is satisfactory but preferably one of inert material to prevent acid reaction with the mold material. Warm air drying at a temperature below 212 F., such as about F. for several hours, is adequate in the various embodiments of the invention to eliminate the water of solution and reaction after removal from the mold. Metal encasement is then optionally applied.

In FIGURE 4, the numeral 7 designates a heating element embedded in a molded body 8 of phosphate insulating material. The heating element is wound in bee-hive shape. As with the embodiment of FIGURE 3, the phosphate mass comprising the body 8 may be formed in a mold of inert material.

FIGURE 5 shows a common brick type heater in which a ceramic body 9 of disc shape has a spiral groove 10 in one face thereof for receiving a heating element 11 in the form of a coiled wire. About the wire heating element 11, an embedding body 12 of magnesium phosphate is formed by filling the groove about the heating element with powdered or granular magnesium oxide and then saturating the magnesium oxide with phosphoric acid to produce a stone-like mass. There may be some reaction between the phosphoric acid and the ceramic body 9 to unite the materials.

FIGURE 6 shows an immersion heater in longitudinal section and including a hollow core 13 of ceramic material on which a resistor wire 14 is wound. The core 13 has two small slots 15, 16 diametrically opposite each other at the closed case end of said core. The core-resistor assembly is inserted in shell 17 of suitable metal, such as aluminum, and magnesium oxide sand 18 is introduced between the outer surface of core 13 and the inner surface of the shell 17. Suitable leads 19, 20, from the resistor winding 14 are admitted through the hollow center 21 of the core 13 and thence through lead holes 22, 23 in a porcelain cap 24 placed inside of the open end skirt 25 of the shell 17. Prior to the placement of cap 24, phosphoric acid solution is introduced into the magnesium oxide in sutficient quantity to saturate the mass. After completion of chemical reaction, the unit is heated to completely dry the embedment.

After assembly of the porcel-ain cap 24, the skirt area 25 may be rolled over the margin of the cap to retain the porcelain in place. It has been determined that the magnesium phosphate embedment has substantially the same expansion characteristics as the resistance element material and the track formed between the resistor wire turns retains such turns against high temperature creep and ultimate short circuit between the turns.

The metallic case or sheath 3, and 17, for example, may be made from aluminum, or these metallic containers may be made from other conventional metals such as copper, iron, iron-nickel alloys, iron-nicke l-chrome alloys, and similar metallic materials. All of these materials react effectively with the phosphoric acid used to form the insulating and heat conductive film on the inner surface of the metallic sheath or case, which film is intimately bonded to the phosphate embedment 1 produced to effectively aid in the heat transfer action from particles of phosphate in the embedment over to particles of the phosphate film on the inner surface of the case and from the case naturally to any material or fluid in which the electrical heater is inserted or with which the heater is associated.

A high Watt density, low inertia immersion heater, as illustrated in FIGURE 7, may be made in a similar manner as the unit in FIGURE 6. In this embodiment, the porcelain, tubular core 26 has a helical groove 27 provided therein to support a helically wound resistor element 40. 1

Wire leads 28, 29 connecting with the ends of the resistor element 40 extend through apertures 30, 31 forward through the center of the ceramic tube, or core 32 and then through the cap 35 at the end of the core. The ceramic core with the resistor element thereon is inserted in a tubular shell 33 of metal, such as aluminum, and centered therein so that the wire resistor element is equally spaced from the side wall of the shell. The shell has one closed end and one open end.

A filling 34 of granular magnesium oxide, or other suitable material reactable with phosphoric acid to form a stone-like material, is placed about the core and resistor element. Phosphoric acid is then introduced, for example into the interior of the tube 32, in suflicient quantity, as indicated hereinbefore, to react with the magnesium oxide and form a stone-like compact mass of slightly porous character. Also, during such reaction some of the phosphoric acid may react with the metal surface of the shell to form a phosphate coating thereon. The excess liquid would then be removed by suction.

A disc 35' of ceramic material is next placed over the end of the core 26 and has apertures 36, 37 therethrough for passing the wire leads 28, 29. The open end of the shell 33 is then rolled inwardly or crimped over the margin of the disc 35 to return the disc. For securing such a heating element in a vessel to be heated, an externally threaded sleeve 38 may be secured about the open end of the shell and may have a radial flange 39 for contacting the Wall of the vessel in seal engagement therewith.

When an embedment is required in a very long element, such as a tubular type, FIGURE 1, I have found that a slight head or pressure on the phosphoric acid solution should be used to aid it to thoroughly infuse the mass.

FIG. 8 of the drawings shows a section of a further modified type ofa heater 41 of the invention. In this instance, the heater 41 is shown in the form of a mica sheet or core 42 about which convolutions of resistance heater wire 43 are provided. The heater 41 also includes a stone-like embedment 44 formed around the edges of the coil of wire 43 on the mica core 42 and also covering the top surface thereof. Hence, a uniform heat conductive embedment can be formed around the end or edge and top surfaces of the resistor wire 43. This provides an improved structure over similar prior art heaters using mica cores. In prior embodiments, frequently the mica cores, like the core 42, have had dead air spaces at the end portions of the wire coil formed so that a non-uniform heating action has been produced.

From the foregoing, it becomes apparent to those skilled in the art that a new and less costly method of solid embedment of electrical heating resistor elements is evolved through the use of chemical reaction. This method provides a new and improved method for readily and conveniently constructing such elements without the use of retaining cements and heavy, expensive equipment for prwsing, swaging or rolling thereby accomplishing the objects of my invention.

Another important objective attained by the invention is the elimination of electrical current leakage in heating elements even after long periods of use whereas, if hygroscopic cements have been employed for the retention of granular type embedment materials, the dielectric strength of the embedment decreases and failure of the heating element, or unit will usually occur relatively rapidly.

In the heater 41, top and bottom mica sheets 45 and 46 are also used. The heater usually has a shell 47 provided therefor.

The effectiveness of the process of the invention and the novel embedment produced is shown by the following comparison:

Leakage Current- Case to Lead Test Voltage Heater A Heater B Heater A is an electrical heater unit made in accordance with this invention whereas Heater B was an identical heater but it had magnesium oxide sand therein bonded together by sodium silicate in accordance with prior practice. Even more striking differences would appear, it is submitted, after use of the heaters because of the hygroscopic nature of the sodium silicate cement in Heater B, if the heaters are exposed to high humidity conditions of, for example, humidity.

This application is a continuation-in-part of my prior application Serial No. 628,272 filed December 14, 1956, now abandoned.

It is obvious to those skilled in the art that the illustrated embodiments of my invention may be varied, changed or modified in combinations other than illustrated or described without departing from the spirit or sacrificing the advantages of the invention, and that my disclosure herewith is illustrative only and my invention is not limited thereto.

I claim:

1. The method of making an electrically insulated heating unit which comprises surrounding a resistor heating element in a closely confined space within a shell with dry particles of a material of a class consisting of fused and ground magnesium oxide, cuprous oxide, alummum oxide, and Zinc oxide which are capable of reacting with phosphoric acid to form a stone-like solid mass, saturating the material with excess phosphoric acid by slight pressure applied thereto and retaining the acid material in contact with the material to form a solid embedment in positive engagement with the resistor elernent by chemical reaction of the acid with the material in situ at room temperature, suctionally removing excess acid, and drying the embedment produced by a positive heating action below the boiling temperature.

2. The method of making an electrically insulated heatmg unit which comprises mounting a metal wire resistor element upon the surface of a ceramic support, intimately and completely surrounding the wire resistor unit in a confined space within a shell with particles of a material of a class consisting of rinsed and ground magnesium oxide, aluminum oxide, cuprous oxide, and zinc oxide which will react with phosphoric acid to form a stone-like solid mass, reacting the material with excess phosphoric acid at ambient temperatures to provide a solid embedment about the resistor element over the support, said wire resistor being inert to said acid, removing any pools of excess liquid present, and removing liquid from the interior of the mass at a temperature above room temperature but below 212 F.

3. The method of making an electrically insulated heating unit which comprises intimately surrounding a resistor heating element in a housing having limited access thereto with dry particles of a material of a class consisting of fused and ground magnesium oxide, cuprous oxide, aluminum oxide, and zinc oxide that are capable of re acting with phosphoric acid to form a stone-like solid mass, said particles being in direct intimate contact with said resistance element and with each other, saturating the material with excess phosphoric acid under pressure to provide a solid embedment in intimate engagement with the resistor element by chemical reaction of the acid with the material in situ at ambient temperature, removing any pool of excess liquid present, and removing liquid from the embedment by heating it :for several hours at about 150 F.

4. The method of making an electrically insulated heating unit which comprises mounting a wire resistor upon the surface of a ceramic support, inserting the resistor element and support into a closely confining tubular aluminum shell having only one open end, surrounding the wire resistor unit in the shell with dry particles of a material of a class consisting of fused and ground magnesium oxide, aluminum oxide, cuprous oxide, and zinc oxide from the open end of the shell, filling the shell with dilute phosphoric acid under some pressure, reacting the material at room temperature with excess phosphoric acid to provide a solid embedment in intimate engagement with the resistor element and the support, said acid also reacting with the metal on the inner surface of the shell to bond the solid embedment thereto, removing any pools of excess liquid present, and removing residual liquid from the interior of the mass by a controlled heating to a temperature above room temperature and below 212 F.

5. The method of making an electrically insulated heating unit which comprises mounting a wire resistor upon an insulating body, closely confining the insulating body in a metal shell made from a class consisting of aluminum, copper, iron, iron-nickel alloys, and iron-nickelchrome alloys, filling the space between the metal shell and the wire resistor and insulating body with a compacted particulate material of a class consisting of fused and ground magnesium oxide, aluminum oxide, cuprous oxide, and zinc oxide to form a solid particulate mass, reacting the material with phosphoric acid in situ at ambient temperature to provide a solid embedment about the element and united to the insulating body, said embedment also being bonded to said metal shell, the phosphor-ic acid being commercial 85 percent acid diluted with about two and one half times its volume with water, the diluted acid being used in substantially equal volume to the amount of said material used and normally forming a pool of acid at an end of said metal shell, suctionally removing any pool of acid present, and removing residual liquid by heating to a temperature of about 150 F. for several hours.

6. The method of making an electrically insulated heating unit which comprises closely enclosing a ceramic support and a wire resistance element thereon within a shell, said metal shell conforming to the shape of said ceramic support and having restricted access space therebetween, filling the space between the shell and the supto) port and thereby enveloping the resistance element with small dry particles of fused magnesium oxide, some of said particles being in direct intimate contact with said resistance element and with each other, applying a phosphoric acid-water solution to the magnesium oxide to saturate it and to react therewith to form a stonelike body about the resistance element, suctionally removing any excess acid-water solution, and heating the body at a temperature above the ambient temperature but below about 212 F. to remove residual moisture therefrom.

7. The method of making an electrically insulated heating unit which consists of closely enclosing a ceramic support and a wire resistance element thereon within a metal shell, filling the shell about the support and the resistance element with dry particles of fused and ground magnesium oxide to envelope the resistance element, applying a phosphoric acid water solution under pressure to the magnesium oxide to saturate it and to react therewith at the ambient temperature to form a stone-like body about the resistance element, said resistance element being inert to said acid, said acid also reacting with the metal on the inner surface of the shell to bond the body thereto, suctionally removing any available excess liquid from the body, and heating the body to a temperature appreciably above room temperature but below 212 F. to remove additional moisture therefrom.

8. The method of making an electrically insulated heating unit which comprises intimately and completely surrounding a resistor heating element in a closely confined space within a housing with ground fused magnesium oxide, said particles being in direct intimate contact with said resistance element and with each other, saturating the magnesium oxide with dilute phosphoric acid under pressure at room temperature to produce a stone-like phosphate mass which is free from voids, suctionally removing any free flowing excess acid present, said heating element being encompassed in said mass and being held in a fixed position thereby, said heating element being inert to said dilute phosphoric acid, and heating above room temperature without any violent evaporation action to remove residual moisture.

9. The method of making an electrically insulated heating unit which comprises intimately surrounding a resistor heating element in a restricted space in a tubular shell having access from only one end thereof with dry particles of a material of a class consisting of fused magnesium oxide, cuprous oxide, aluminum oxide, and zinc oxide, and capable of reacting with phosphoric acid to form a stone-like solid mass in intimate and continuous contact with the resistance heating element, filling the shell around said resistance heating element with dilute phosphoric acid, reacting the material with excess phosphoric acid at the ambient temperature to provide a solid embedment about the resistor element, suctionally removing any free-flowing excess acid, and heating above room temperature without any violent evaporation action to remove residual moisture.

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